Abstract

The West African Craton is defined by the presence of Archaean and Palaeoproterozoic (Birimian) rocks. Since 1990, researchers have published several papers proposing various regional scale accretionary models for the evolution of the Birimian series and the Eburnean orogeny. The Birimian lithostratigraphic succession starts with a major sequence of tholeiitic pillow basalts with intercalations of sediments, overlain by detrital and carbonate sediments, associated with calc-alkaline volcanics and plutons. A global scheme for the geodynamical evolution can be reasonably constrained using the characteristics of early tholeiitic plutono-volcanic and granitoid rocks. Geochemical, geochronological and structural characteristics of Palaeoproterozoic magmatic rocks of the Man-Leo Shield distinguish three tholeiitic (PTH1, PTH2, PTH3) and two granitic (PAG, PBG) series and date three principal events that have characterized the 2250–2000 Ma period: (1) Event I (2250–2200 Ma) characterized by tholeiitic volcanism with a widespread first tholeiite generation (PTH1), interpreted as the eruption of a mantle plume in an oceanic basin floor; (2) Event II (2200–2150 Ma) marked by second (PTH2) and third (PTH3) tholeiite generations and a calc-alkaline magma setting; high granitization and amphibole-bearing (PAG) granitoid emplacement with greenstone belt deformation. This event has been favoured by a crustal subsidence in a broad synclinorium mega-structure followed by vertical tectonics; (3) Event III (2150–2000 Ma) characterized by main biotite±muscovite (without amphibole) bearing (PBG) granitoid emplacement in a context of transcurrent deformation. There are many similarities between West African Palaeoproterozoic rocks and Archaean formations and their crustal evolution also may be similar.

The West African Craton (Fig. 1) is made up of two main areas of Archaean and Palaeoproterozoic rocks: the Reguibat Shield in the north and the Man-Leo Shield in the south. Some isolated Palaeoproterozoic rocks are found between the two shields within Neoproterozoic formations that are widespread in the east of the craton. In the Man-Leo Shield, the Palaeoproterozoic comprises Birimian terranes (2.2–2.0 Ga) (Abouchami et al. 1990; Boher et al. 1992; Taylor et al. 1992; Hirdes et al. 1996), which predominantly comprise granitic batholiths, plutono-volcanic and meta-sedimentary greenstone belts. Geodynamic evolutionary models have been proposed in reference to two concepts: (1) modern tectonics, for example horizontal plate tectonics, and (2) vertical movements related to differential gravity of magmas such as those described for some Archaean crustal evolution. For West African Palaeoproterozoic rocks, geochemical data interpreted with respect to Phanerozoic tectonic discriminantary diagrams indicate variable settings for the same data in the same area. For instance, Leube et al. (1990) proposed a continental rift for the Palaeoproterozoic rocks in Ghana, whereas arc or subduction related contexts have been often proposed in Mali (Liégois et al. 1991), Ivory Coast (Vidal & Alric 1994), Niger (Ama Salah et al. 1996; Soumaila et al. 2004), Burkina Faso (Béziat et al. 2000), Senegal (Ndiaye et al. 1997; Diallo 2001) and Guinea (Egal et al. 2002). For the entire craton, Abouchami et al. (1990) and Boher et al. (1992) have proposed mantle plume derived magmas on oceanic plateaus to explain the early stage of Palaeoproterozoic tholeiitic series emplacement, followed by island arcs on the top of the oceanic plateaus that then collided with the Man Archaean craton. For the Ivory Coast, Vidal et al. (1996) proposed a geodynamic scheme where juvenile tholeiitic magmas form in an oceanic context, followed by gravitational deformation and transcurrent shearing, with granitoid pluton emplacement and vertical isostatic readjustment; they argued this model in respect to Archaean crustal development. In Burkina Faso, Béziat et al. (2000) in a study of ultramafic-mafic and associated volcanic rocks, suggested that the Palaeoproterozoic crust of the West African Craton was heterogeneous and not the consequence of a single process of genesis; they proposed both volcanic arc and oceanic plateau accretion. Nevertheless, isotopic data from all the West African Craton (Abouchami et al. 1990; Liégeois et al. 1991; Boher et al. 1992; Taylor et al. 1992; Cheilletz et al. 1994; Hirdes et al. 1996; Ama Salah et al. 1996; Ndiaye et al. 1997; Dia et al. 1997; Doumbia et al. 1998; Gasquet et al. 2003), indicate a juvenile source for the widespread Palaeoproterozoic volcanic and granitic rocks.

The present study uses geochemical analysis of mafic and granitic igneous rocks from Burkina Faso, considered in the light of published geochemical data from elsewhere in the Man-Leo Shield and their structural setting, to shed light on the apparently conflicting models for the evolution of the Birimian terranes and to propose a new model for their geodynamic evolution that critically considers development of uniformitarian plate-tectonic processes within the craton.

Early deformation and related plutons

Early syn-metamorphic deformation is preserved in the greenstone belts by a sub-vertical schistosity developed across the entire shield: Senegal (Bertrand et al. 1989; Pons et al. 1992), Burkina Faso (Lompo et al. 1991; Lompo 1991), Niger (Pons et al. 1995), Ivory Coast (Vidal et al. 1996; Caby et al. 2000). This regional sub-vertical foliation is associated with a down-dip plunging lineation due to a general plane strain deformation. Structures in some areas of the shield indicate that shortening is >50%, synchronous with a steeply plunging to sub-horizontal stretching lineation (Lompo 1991; Pons et al. 1992, 1995; Caby et al. 2000). In the greenstone belts, the foliation is variably expressed because of the differences in material rheology (association of meta-volcanic and meta-sedimentary rocks). The regional foliation can display variable dips in some areas (Vidal 1987). The strike trajectory (Fig. 3a, b) is sinusoidal in accordance with the belt strike and pluton's shapes, or locally disturbed by the late deformation.

Emplacement mechanism studies of layered or banded early granitoids (PAG) have been carried out in some areas of the shield (Dupuis et al. 1991; Pons et al. 1992, 1995; Vidal et al. 1996; Caby et al. 2000). In the early granitoids (PAG), fabrics are S–L type, oriented in concordance with the schistosity in the greenstone host rocks, especially at the contact zones with high-grade metamorphic facies proving their syntectonic emplacement mechanism (Pons et al. 1995; Gasquet et al. 2003). According to these authors, sub-vertical stretching lineations are related to magma ascent during the granitoid emplacement and the gently plunging lineations are related to the main shear zones. Indeed, pressure and temperature (P–T) conditions obtained in some widely-spaced areas (Fig. 3a) on the shield [south Ghana (John et al. 1999; Klemd et al. 2002), west Ivory Coast (Caby et al. 2000), north Burkina Faso (Debat et al. 2003) and east Burkina Faso (own observations)], give the same results at 5–6 kbar at 500–700 °C, similar to the conditions retrieved from the aureoles of the greenstone host rock as in the amphibole-bearing granitoids (PAG). These observations suggest that most of the PAG ganitoids are syntectonic intrusives in the greenstone terranes; the structural patterns of greenstone belts are indicative of a vertical tectonic regime resulting from the interference between the dynamics and thermal effects of pluton emplacement under the well-known NW–SE shortening that affected the entire shield (summarized in Gasquet et al. 2003).

Geochemistry

Methodology

Geochemical data are selected in respect to rock type and well-described lithologies (published and unpublished data) showing the least alteration. Palaeoproterozoic rocks in the greenstone belts are more or less affected by hydrothermal and/or meteoric water alteration. The selection of samples took into account the petrographic description, focusing on the less altered rocks and seeking a representative set of samples for each type of rock. Geochemical data of 215 samples with SiO2 =35–65% in the greenstone belts and 223 samples with SiO2=52–78% in the granitoids, are from Burkina Faso, Ivory Coast, Ghana, Niger, Senegal and Guinea. In the tables, data are presented with averages but plots on diagrams are clusters of representative positions for Burkina Faso and average positions for the other areas in the Man-Leo Shield. Major (wt%) and trace elements used for rocks discrimination have been plotted but only the more significant ones are considered in this study. SiO2, TiO2, Al2O3, FeO (Fe2OT3 or FeOT), MgO, Ni, Ce and REE are used for the characterization of plutono-volcanic series in the greenstone belts and SiO2, TiO2, Al2O3, FeO (Fe2OT3 or FeOT), MgO, CaO, Na2O, K2O, Rb, Ba, Sr and REE for granitoids.

Major elements concentrations suggest an evolution of a magmatic suite with tholeiitic, ultramafic and calc-alkaline rocks differentiation. In that magmatic suite, PTH1, PTH2 and PTH3 series have specific REE characterization and are more or less distinct according to their relative content in SiO2, Al2O3, FeO and MgO. Each subgroup can be represented by a field on an FeO/MgO v. SiO2 diagram (Fig. 4). For the same ratio FeO/MgO, the PTH3 subgroup has higher SiO2 values (50%<SiO2>55%) than PTH2 (45%<SiO2> 50%), whereas PTH1 is represented in the entire tholeiites (45% <SiO2> 55%). This distinction, through major elements, is in accordance with REE variation.

In respect to REE patterns, tholeiites intrusive rocks (PTHI) are close to PTH1 lavas. Calc-alkaline lavas (PCAL) and intrusive rocks (PCAI) and (PUI) are similar to PTH3, except that PUI are less enriched in REE than PTH3. In respect to Miyashiro's (1974) discriminant boundary (Fig. 4), Béziat et al. (2000) showed that the ultramafic rocks (PUI) used in this work, according to their cumulative nature and the association with differentiated tholeiitic gabbros, appear linked to the calc-alkaline suite.

Geotectonic context of tholeiite emplacement

The tectonic context of the emplacement of PTH1, PTH2 and PTH3 is discussed in comparison between REE profiles and Al2O3, FeO contents (Fig. 6): (a) PTH1 with flat REE profiles are comparable to oceanic plateau basalts and Al2O3 content is similar to oceanic island basalts (OIB), although they are higher in FeO content; (b) PTH2, with weakly LREE-depleted profiles, are comparable to oceanic NMORB basalts but differ by having lower Al2O3 and higher FeO contents; (c) PTH3, with LREE-enriched profiles and higher Al2O3 content, are comparable to arc tholeiites but they have higher FeO contents; (d) in comparison to REE profiles of modern Pacific Ocean basalts (Storey et al. 1991) PTH1 and PTH2 are similar to rocks from the Nauru Basin (1 & 3), Ontong-Java (2) and Manihiki (4) oceanic plateau basalts; (e) in general, the Palaeoproterozoic tholeiites (PTH) have Al2O3 contents similar to OIB and some Archaean tholeiites from Canada and Australia; FeO content of PTH is more than OIB but less than Archaean tholeiites. In respect to Phanerozoic tectonic discriminant diagrams (Fig. 7) of Sun & McDonough (1989) and Jochum et al. (1990), Palaeoproterozoic tholeiites (PTH) have a similar evolution as Archaean tholeiites. According to SiO2 content, PTH are MORB-like but differ by having higher Ni and FeO content. Their lower contents of incompatible (Ce) and compatible (Ti) elements make PTH closer to island-arc volcanic rocks (IAV) but IAV have very low FeO and Ni content. Al2O3, FeO and Ni of PTH plots are common in the OIB field and suggest that the OIB geodynamical context is the best match for the PTH.

The three subgroups of tholeiites present three REE patterns, suggesting three different contexts of geodynamic setting. The principal characters of PTH with high FeO and lower Al2O3 content make them closer to modern oceanic island basalts (OIB), and REE profiles of the main tholeiites (PTH1 and PTH2) are similar to modern oceanic plateau basalts. Thus it is possible to imagine an oceanic basin setting for the primary PTH emplacement confirming the first stage of tholeiites emplacement proposed by Abouchami et al. (1990) and Boher et al. (1992). However, some of the tholeiites, especially PTH3, with lower FeO and REE patterns similar to modern arc tholeiites (Fig. 6c), may have a different tectonic context of emplacement and therefore should be related to the next stages of dynamic evolution discussed below.

Magmatic source of tholeiites

In the Al2O3 v. Mg# diagram (Fig. 7), Archaean rocks and modern OIB show two trends; according to Arndt (1998), the first trend of Al2O3 increasing with Mg# decreasing is characteristic of magnesium lavas, and the second trend of constant to decreasing Al2O3 with decreasing Mg# is characteristic of more highly developed magmas. PTH rocks are in accordance with the second trend and can be interpreted as magma fractionates of a plagioclase‐bearing assemblage. According to the experimental work of Hirose & Kushiro (1993) on the melting rate of mantle peridotites, the depth of melting can be related to Al2O3 content. Mantle melting at low pressure produces Al2O3-rich magmas while melting at deeper levels results in Al2O3-poor melts. The experimental work of Langmuir & Hanson (1980) also shows that FeO content is directly related to depth of partial melting of the mantle. With the highest FeO content in the Palaeoproterozoic tholeiites (PTH1 and PTH2) with low Al2O3 content with respect to modern basalts, suggests that the main PTH are derived from partial melting from a deep source in the mantle similar to modern oceanic island basalts or Archaean tholeiites.

PTH3 displays higher content of Al2O3 and lower FeO content than other Palaeoproterozoic tholeiites; it is suggested that this magma derives from partial melting at a higher level in the mantle. On the other hand, their characteristics are close to island arc basalts (Fig. 6c) which suggests that the melt source could be hydrated and probably metasomatized. In a modern context, this can be explained by a subduction system where the top of the plate (in contact with seawater) participates in the melting. However, isotopic data from Abouchami et al. (1990), Taylor et al. (1992), Cheilletz et al. (1994), Hirdes et al. (1996), Ama Salah et al. (1996) and Dia et al. (1997), suggest a juvenile source for the Palaeoproterozoic tholeiites of the West African Craton. PTH3 could be derived from the partial melting of the top of the mantle which may be hydrated and metasomatized.

Béziat et al. (2000) undertook a petrogenetic study of mafic-ultramafic and calc-alkaline rocks, where it was possible to establish field relationships and mineral analysis, and concluded that the cumulate ultramafic rocks (PUI) and related gabbros used in this work, are derived from fractional crystallization of PTH-like basic magma.

Granitoids series and petrogenesis

Palaeoproterozoic granitoids of the Man-Leo Shield comprise two types distinguished by mineral composition, principally by the presence (PAG) or absence (PBG) of amphibole. This distinction is also outlined by the variation of chemical major and trace (REE) elements (Table 2). In the Q–A–P diagram (Fig. 8a), PAG (amphibole-bearing granitoids) with SiO2=60–70% comprises diorites, tonalities and granodiorites. In the K2O/Na2O diagram (Fig. 8b), their composition is mainly tonalite/trondhjemite and granodiorite. PBG (biotite±muscovite-bearing granitoids) with SiO2= 70–75% comprising granodiorite and monzogranite (Fig. 8a), or simply granite, granodiorite and adamellite (Fig. 8b). An oxide v. SiO2 diagram (Fig. 9a) shows that PAG and PBG constitute two magmatic suites with constant Al2O3 (c. 15%), whilst the other elements (TiO2c. 0.5%; CaO c. 4%; Fe2O3c. 4% and MgO c. 2%) in the PAG, and (TiO2<0.3%; CaO c. 1.5%; FeO c. 2% and MgO c. 0.5%) in the PBG, are inversely proportional to SiO2. REE (Fig. 10) are weakly fractionated (2<LaN/YbN>40) in the PAG, whereas they are highly fractionated in the PBG (3<LaN/YbN>165).

According to major chemical elements (Fig. 8a, b), Palaeoproterozoic granitoids (PAG and PBG) of the Man-Leo Shield constitute a medium potassic, calc-alkaline to calcic magmatic suite with PBG purely calcic. For the same Na2O content, PBG ganitoids are more K2O-rich. The two series are clearly distinguished by their isotopic age evolution (Fig. 2b) and the fractionation of REE.

According to the ratio LaN/YbN v. YbN (Fig. 11a), the Palaeoproterozoic granitoids (PAG and PBG) have similar characters as Archaean TTG (Tonalite–Trondhjemite–Granodiorite), with a high fractionation of REE (LaN/YbN=3–160) indicating an enrichment of LREE and depletion of HREE. For Jahn (1998), these characteristics suggest fractionation of plagioclase±hornblende± pyroxene during fractional crystallization or partial melting at low pressure (≤ 5 kbar). Some geochemical experiments indicate that Archaean TTG are produced from partial melting of garnet-bearing amphibolite or quartz eclogite with a residue dominated by amphibole and/or garnet-bearing (Arth & Hanson 1975; Barker & Arth 1976; Glikson 1979; Jahn et al. 1981; Martin et al. 1983; Jahn & Zhang 1984; Condie 1986; Shirey & Hanson 1986; Drummond & Defant 1990). In the Sun & McDonough (1989) diagram (Fig. 11a), REE plots of PAG and PBG are in accordance with curves of liquid composition from eclogite or 25% garnet-bearing melt. Variation of Rb, Sr and Ba is compatible with partial melting followed by fractionated crystallization for PAG and PBG; in the Rb/Sr v. Rb and Ba v. Sr diagram of Martin (1985) (Fig. 11b, c), PAG are preferentially in the field of partial melting (PM) of eclogite or mantle, whereas PBG are mainly in the field of fractionated crystallization (FC).

A model of geodynamic evolution

The widespread Palaeoproterozoic magmatic rocks that are common throughout the entire Man-Leo Shield are characterized by a predominance of tholeiitic magmas and abundance of layered or foliated granitoids. Greenstone belts are straightened and have a schistose fabric imposed on them during early granitoid emplacement. Tholeiites frequently display flat REE patterns or have enriched LREEs, low Al2O3 and high FeO content in comparison with actual tholeiites. They also have high SiO2 and low Ce and Ti, making them similar to island arc volcanic rocks. Granitoids have highly fractionated REEs (La/Yb)N with low values of (Yb)N. εNd is positive (c. +2) (Dupré et al. 1984; Shirey & Hanson 1986; Machado et al. 1986; Abouchami et al. 1990; Boher et al. 1992) and suggests evolution of a juvenile crust. Some Archaean granite-greenstone belts comprise petrographical and geochemical similarities with Palaeoproterozoic rocks of the Man-Leo Shield and are thought to have been formed by mantle plume processes (Dostal & Mueller 1997; Hollings & Wyman 1999; Hollings et al. 1999; Wyman 1999; Tomlinson & Condie 2001; Wyman et al. 2002; Sandeman et al. 2006). In respect to these authors, tholeiites with flat REE patterns and ultramafic rocks (Archaean komatiites), are commonly related to mantle plumes; even associated basalts with oceanic island and oceanic arc basaltic characteristics, are termed arc plume associated greenstone belts. This study shows that the Palaeoproterozoic (Birimian) rocks of the Man-Leo Shield show similarities with Archaean terranes and lack the features that typify Phanerozoic subduction settings. These Palaeoproterozoic rocks may therefore be formed by processes similar to those which generated the Archaean rocks.

Subduction or arc models proposed for the Palaeoproterozoic rocks of the Man-Leo Shield are not well constrained for many reasons: (i) according to chemical data, there is no contribution of older rocks in early Palaeoproterozoic tholeiitic magmas (Abouchami et al. 1990; Boher et al. 1992); (ii) according to field relationships, there is no expressed metamorphic zonation across the belts, and there are no lithospheric-scale thrusts described anywhere in the entire shield. Instead, the Palaeoproterozoic belts of the Man-Leo Shield attest to homogeneous metamorphic conditions along horizontal profiles through the shield and trace out ‘dome and basin’ geometries with extensive domains of flat-lying fabrics with stretching lineations and vertical shear zones; these structural and metamorphic patterns have been observed in many Archaean and Palaeoproterozoic belts (Cagnard et al. 2007), suggesting the particularity of deformation modes of weak lithospheres under compression. The spatial extent of these rocks (>500 km in a NW–SE cross‐section on the shield), after considerable tectonic flattening, argues against the subduction of a unique plate, producing magmas with the same characters across the entire shield, or the juxtaposition of many small subducted plates similar to Phanerozoic tectonics.

The first stage of accretion can be considered as mantle plume evolution in an oceanic basin context, in accordance with Aboucahmi et al. (1990) and Boher et al. (1992). On the other hand, the next stages producing late tholeiites (PTH3) and calc-alkaline (PCA) magmas with arc-related signatures, appear contrary to the rheological and structural framework described above. I suggest that similar to the Archaean evolution, the lithosphere during the Palaeoproterozoic behaved in a highly ductile manner in the Man-Leo Shield under regional NW–SE shortening providing widespread PAG granitoids (Pons et al. 1995; Vidal et al. 1996; Gasquet et al. 2003). This resulted in widespread folding rather than faulting. So subduction models cannot easily explain vertical tectonics and foundering of oceanic crust, nor in the first stage of accretion (Ama Salah et al. 1996), nor in the next stages with crustal thickening (Boher et al. 1992).

The geodynamic model proposed in this chapter emphasizes the latter stages of magmatism that followed the emplacement of tholeiites. It is based on the ductile behaviour of the lithosphere at this time, which as it was shortened, foundered by subsidence processes instead of breaking and sinking into the asthenosphere as a rigid plate by subduction processes.

Geodynamic evolution of the Palaeoproterozoic magmatic rocks

The geochemical, geochronological and structural characteristics of the Palaeoproterozoic magmatic rocks, allow the timing of the principal magmatic events to be worked out. The subdivision is not strict, but outlines the periods of intensive activity; according to these dates, there are both syn- and post-volcanic granitoid emplacement. Three principal magmatic events (Fig. 12) have characterized the 2250–2000 Ma period: (a) Event I (2250–2200 Ma) is characterized by tholeiitic volcanism with a widespread PTH setting by predominant mantle plume evolution; (b) Event II (2200–2150 Ma) is marked by PAG granitoid emplacement with greenstone belt deformation in a calc-alkaline magma setting; vertical tectonics characterize this event; (c) Event III (2150–2000 Ma) is characterized by PBG granitoid emplacement in a context of transcurrent movements.

Studies on the Palaeoproterozoic tholeiitic pillowed basalts and pelagic sedimentary rocks (pelites and carbonatites) on the Man-Leo Shield (Abouchami et al. 1990; Boher et al. 1992; Bossière et al. 1996; Pouclet et al. 1996) suggest that emplacement occurred in a marine basin. The presence of a banded iron formation in the eastern margin of the Man Archaean nucleus confirms an oceanic setting (Fig. 13a) for Palaeoproterozoic rocks in this area. Without data on the mechanism of formation of this ocean between two Archaean nuclei, we can consider a ‘proto-oceanic crust’ with a near mantle composition, similar to oceanic lithosphere, where the top is in contact with seawater and could be hydrated and metasomatized. This top surface could present some rare sedimentary fragments derived from continents.

Event II (2200–2150 Ma)

Subsidence and vertical movements.

c. 2200 Ma – Beginning of the subsidence

PTH3 magmas derived from partial melting of the top of the mantle (cf. Magnetic source of tholeiites, above). Without structural weaknesses such as suture zones to explain the plunging of the oceanic plate, we can reasonably imagine another mechanism such as subsidence of the basin floor (Fig. 13b-E.II) due to tholeiitic (PTH1 and some PTH2) volcanism. A depression by subsidence (Gorman et al. 1978) could thus allow the upper part of the mantle to attain the deeper mantle temperatures. Favourable arguments are: (1) the presence of underlying plumes with material escape; (2) the quantity of plutono-volcanic rocks (widespread thick tholeiitic rocks known across the entire shield) formed during the period of accretion; the overload of this material could create a balance of density favourable to depression with basin margin convergence. These arguments show that the mega-structure has subsided.

c. 2180–2150 Ma. Evolution of the subsidence

The depression of the mega-structure would favour the partial melting of the base of the pile and also refold the edges of the synclinorium (Fig. 13b-E.II). This melting could produce PTH3-type and calc-alkaline magmas widespread across the entire shield and considered by all workers as posterior to PTH1 and PTH2 types. The mantle signature of early granitoids (PAG) and the other characters (cf. Granitoids series and petrogenesis, above) show that PAG derived from partial melting of the top of mantle and probably a part of underlying basaltic pile. At this stage, the main deformation is dominated by vertical movements due to upwelling granitic magma. The global NE–SW preferential orientation of the major structures (cf. Early deformation and related plutons, above) in the greenstone belts and early granitic batholiths (shapes and magmatic fabrics) suggests that the whole system evolved during NW–SE shortening.

Conclusion

An overall scheme for geodynamic evolution of the Man-Leo Shield during the Palaeoproterozoic can be reasonably constrained using the characteristics of early tholeiitic plutono-volcanic and granitoid rocks. Geochemical, geochronological and structural characteristics of these rocks allow us to date three principal events which have characterized the 2250–2000 Ma period in the whole shield. The proposed schematic model of geodynamic evolution is a simplified presentation and scales are not respected. Nevertheless, chemical and structural constraints have been taken into account.

The characteristics of the Archaean and Palaeoproterozoic rocks and tectonic context present very close affinities. It seems that there is no fundamental change in crustal evolution from the Archaean to the Palaeoproterozoic period in this region.

Acknowledgments

This synthesis has been prepared with respect to the research topics of the IGCP 509 project. I gratefully acknowledge Project Leaders for this initiative. Thanks to David Evans and particular thanks to Mark Jessell and Alan Collins for help to improve the presentation of the document. Thanks to referees and anonymous contributors.

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